Immune-mediated polyarthritis in dogs: symptoms and care

Overview

In IMPA, inflammation affects the synovial membrane lining the inside of the joint. More fluid than normal collects in an inflamed joint. Several joints are often affected fairly symmetrically. This differs from ordinary osteoarthritis, a degenerative joint disease.

In non-erosive disease, X-rays do not usually show bone-eroding changes. In erosive disease, joint cartilage and the underlying bone may be destroyed. The non-erosive form is more common. The disease is considered rare, but vague symptoms may make it difficult to recognise.

Joint fluid surrounds the joint surfaces and helps protect them during movement.

Causes

An immune reaction starts in the wrong place

An immune complex consists of an antibody and the antigen bound to it. An antigen is a substance the immune system recognises as foreign. Complexes accumulating in the synovial membrane trigger a build-up of inflammatory cells. Substances and enzymes released by white blood cells disrupt the protective function of joint fluid.

Antibodies are proteins produced by white blood cells.

Primary and reactive disease are distinguished

In primary, or idiopathic, disease, no triggering underlying condition is found. This accounts for about 50–65% of cases. Reactive disease may follow an infection elsewhere in the body. This type has been described as accounting for 13–25%.

An underlying cause may be a urinary tract infection or inflammation of the heart's inner lining, for example. Intestinal or liver disease can also trigger reactive joint inflammation. In tumour-associated disease, the tumour is outside the joints. Some medicines, such as sulfonamides, may be associated with reactive disease.

Systemic lupus erythematosus, or SLE, is an autoimmune disease affecting several organ systems.

Polyarthritis may be part of more widespread SLE. Alongside the joints, it may affect the skin, blood cells or kidneys, for example.

Risk factors

The disease is often diagnosed in young adult dogs, typically at 2.5–4.5 years of age. However, younger and older dogs are also affected. In a study of 73 dogs with primary disease, median age was 4 years and 6 months. Age ranged from 7 months to 9 years and 6 months.

Both small and large dogs can develop the disease. In one study, 62% of patients were small dogs. No clear overall sex predisposition has been demonstrated. Some breeds have distinct syndromes in which polyarthritis is one feature.

An individual case has been described in a puppy as young as 2 months.

Typical symptoms

Pain may cause a very stiff or cautious gait. A dog may be lame on one leg and later on another. Getting moving after rest is often difficult.

• Lameness shifts between legs or affects several limbs.

• Your dog avoids walking and is reluctant to get up.

• The wrist or hock area may swell.

• Joints feel warm and are painful to touch.

• Your dog has a fever and is more tired than usual.

• Appetite decreases.

• Your dog may lose weight.

• The neck or back may be painful.

About 40% of affected dogs have no joint swelling. Some have only general symptoms without obvious lameness.

Complications and consequences

Inflammation may recur after a good initial response

The outlook ranges from fair to good depending on the form of disease. In one follow-up study of 73 dogs, 69, or 95%, initially responded favourably to treatment. Lasting remission without ongoing medication was achieved in 46 dogs, or 63%. These dogs were followed for 446–2,831 days after medication was stopped.

During or after the first course of treatment, symptoms recurred in 35 of 73 dogs, or 48%. Recurrence has also been described in 28% in other follow-up studies. Another reported recurrence rate is 44%. In long-term follow-up, IMPA was considered the reason for euthanasia in 14 dogs, or 19%.

Severe erosive inflammation may permanently damage joint structure. Wrist or hock ligaments may become lax. Kidney problems or ligament laxity can worsen the outlook.

Significant protein loss in urine has been found in about 20% of dogs with IMPA. This means a urine protein-to-creatinine ratio, or UPC, above 2. A further 18% had a UPC of 1–2. In erosive disease, a joint may become unstable or dislocate.

Diagnosis at the vet

Joint fluid sampling identifies inflammation

The vet reviews the history and examines painful joints. Joint fluid analysis is central to diagnosis. Samples are usually taken from at least 3 joints. Besides the wrists and hocks, a knee may be sampled, for example.

Sampling is usually performed under sedation or general anaesthesia. A small needle enters the joint to collect fluid. Fluid from an inflamed joint may be thin and cloudy. It usually contains many white blood cells called neutrophils.

Joint infection can cause similar cell changes. Bacterial culture is therefore also performed. Direct blood agar culture of a sample from a septic joint may be negative in 50–70% of samples.

Healthy joint fluid usually contains fewer than 3,000 nucleated cells per microlitre. Fewer than 10% are neutrophils. In IMPA, neutrophils are often non-degenerate, meaning their structure shows no degenerative changes. Similar inflammation can occur in both erosive and non-erosive disease.

Underlying disease and the nature of joint damage are investigated

Blood and urine tests help look for systemic disease. The inflammatory marker CRP may be raised in IMPA. Changes in it later help monitor treatment response. X-rays look particularly for bone-eroding changes. Early erosive changes are not always visible on the first images.

Chest and abdominal imaging may be needed to find an underlying cause elsewhere. Heart ultrasound can investigate possible endocarditis. Tests for tick-borne diseases are chosen according to exposure and travel history. Borrelia antibodies indicate exposure but do not confirm that it caused the joint symptoms.

Cerebrospinal fluid may be tested in a dog with neck or back pain for concurrent steroid-responsive meningitis-arteritis, or SRMA. SRMA is an inflammatory syndrome affecting the meninges and arteries. Inflammation of the spine's small facet joints can also cause back pain.

Urine may also be cultured for bacteria. Antinuclear antibody, or ANA, testing may be used if SLE is suspected. A positive result needs supporting symptoms consistent with the disease. Poor treatment response or recurrence may prompt repeat joint imaging.

In one imaging study, 4 of 13 dogs undergoing heart ultrasound had changes consistent with endocarditis. Only 2 of those 4 had an audible heart murmur.

Blood samples may show mild non-regenerative anaemia. Blood albumin may also be low.

Treatment

Treatment targets the form of disease

The vet plans treatment according to whether the disease is primary or reactive. Primary disease is treated with medicines that suppress the immune response. Common first-line medicines are corticosteroids such as prednisolone or prednisone. An untreated infection prevents immunosuppressive treatment from being started.

In reactive disease, the underlying trigger is treated or removed. A tick-borne infection requires appropriate antimicrobial treatment. For a drug-related reaction, the vet stops or changes the suspected medicine. In some dogs, treating the underlying disease alone resolves the joint symptoms.

Response and adverse effects guide ongoing care

The first symptom relief may be seen within 48 hours.

A clinical response in primary disease is usually achieved within 2–3 weeks. Recurrence, an incomplete response or marked adverse effects may require another immunosuppressive medicine. Options include ciclosporin, azathioprine or leflunomide. Mycophenolate is also used in some circumstances.

Some dogs eventually become symptom-free without medication. Others need long-term or lifelong maintenance treatment. Reducing treatment too quickly can trigger recurrence. Do not suddenly stop a corticosteroid yourself, as this can cause a dangerous hormone deficiency.

Corticosteroids can increase thirst. Urine production may also increase. Appetite may grow. Suppression of immune defences increases infection risk.

Veterinary examinations and blood tests monitor treatment. Repeat joint fluid samples can assess remission, when inflammation has settled. CRP can be monitored alongside clinical symptoms.

Corticosteroids can cause panting. Muscle strength may decrease during treatment. Digestive tract ulcers are a possible adverse effect. Wound healing may also slow down.

Checking CRP 1–2 weeks after starting treatment may help predict ongoing treatment needs.

Rest and supportive care help during the severe early phase

Severe general symptoms may need urgent emergency care. Intravenous fluids may be necessary. Nausea can be treated separately. Pain relief may include gabapentin or opioids, for example.

Activity is restricted during the acute painful phase. Exercise is later restarted gradually. A suitable body weight reduces loading on damaged joints. Severe ligament laxity may sometimes require joint fusion surgery.

Prevention

Primary immune-mediated polyarthritis is difficult to predict or prevent. Annual health checks and screening blood tests may help find systemic disease early. Preventing infections and parasites reduces some inflammatory triggers.

After immune-mediated disease, future vaccinations are planned with the vet. Possible inherited susceptibility is also a reason to collect family health information. Some breed clubs advise reporting serious autoimmune disease to the breeder and the club's breeding committee.

Open sharing of diagnostic health information supports breeding healthier dogs.

Important for dog owners

Record which leg is lame, how your dog gets up and how far it wants to walk. Take a short comparison video on level ground if your dog can move without being forced. Note appetite, drinking, urination, any fever and day-to-day variation too.

Keep an up-to-date list of medicines, supplements, vaccinations and travel. Agree whom to contact if lameness or fever returns. Ask for a written follow-up and medication adjustment plan. Report increased thirst, digestive symptoms and new signs of infection before changing treatment yourself.

Wash your hands after handling corticosteroid tablets. Anyone pregnant or who might be pregnant should avoid handling them.

Frequently asked questions

Is polyarthritis the same as osteoarthritis?

No: in immune-mediated polyarthritis, the immune system triggers inflammation directed at the joints. Osteoarthritis is a different, degenerative joint disease. IMPA can cause pain in several joints and general symptoms such as fever and loss of appetite. Inflammation can be confirmed in fluid samples collected from several joints.

Can my dog have IMPA without swollen joints?

Yes: about 40% of affected dogs have no joint swelling. Sometimes the disease appears mainly as general symptoms. Investigating unexplained fever may therefore require joint fluid samples even when lameness or joint enlargement is not obvious. Early changes may be very subtle.

Why must joint fluid be tested before treatment?

A joint fluid sample shows inflammatory cells and helps distinguish an immune reaction from infection. Bacterial infection and IMPA can cause similar cell changes. Infection prevents immunosuppressive treatment from being started. Samples are usually collected under sedation or general anaesthesia.

Can my dog recover completely?

Some dogs achieve lasting remission without ongoing medication. In a follow-up study of 73 dogs with primary disease, 46, or 63%, did so. Others need lifelong maintenance treatment. Symptoms may recur even after a good initial response.

Should I stop steroids if my dog drinks more?

Increased thirst and urination are possible corticosteroid adverse effects. Suddenly stopping them yourself can cause a dangerous hormone deficiency. The vet adjusts ongoing care according to disease control and adverse effects. Corticosteroids may also increase appetite. Panting can occur too.